Manatsu Natsumi is a first year middle school student who alongside her family has moved to Aozora City from Minamino Island. She begins her new life there and her first semester in middle school until one day, she met with a strange person named Laura, a mermaid who had originated in a mystical place called Grand Ocean, the land of Mermaids. She has been searching for legendary warriors in her legends who can stand up against the Dark Witch from the bottom of the ocean that took over her nation, and that she lost her motivation to fight back after the takeover. But as one of the witch's minions has found Laura after her escape, Manatsu suddenly is blessed with the powers of the sea that allows to her to become Cure Summer, one of the legendary warriors foretold in the legends of the Grand Ocean.